In today's advancing semiconductor manufacturing industry with increased levels of integration, integrated circuits are more commonly being formed to include both analog and digital circuits on the same chip. When both analog and digital circuits are placed on the same chip to form an integrated circuit, there is significant parasitic coupling between the two types of circuits. As a result, the performance of the circuits is degraded. It is therefore desirable to minimize such coupling. Integrated circuits may also be formed to include various analog circuits on a chip which may advantageously be isolated from one another. As an example, an analog transmitter circuit formed on a chip may generate enough power to saturate an analog receiver circuit formed on the same chip and render the receiver useless. It is therefore also desirable to isolate analog circuits from other analog circuits formed on the same chip to prevent such undesirable parasitic coupling. Generally speaking, it is desirable to provide both DC and RF isolation to the various analog and digital components placed on the same chip and which combine to form an integrated circuit.
Previous attempts to minimize parasitic coupling and to isolate the various analog and digital components from each other include low resistance buried layers formed by diffusion or implantation, and silicon-on-insulator substrates with oxide-filled trenches. The low resistance buried layers, however, lack vertical conducting surfaces and therefore only provide partial lateral isolation. Moreover, the resistivity of materials conventionally used to form the buried layer, is substantially higher than that of superior conductors such as metal, so these conventional materials provide less than optimal RF shielding. The silicon-on-insulator substrates with oxide-filled trenches provide good DC isolation, but the dielectric isolation material is the perfect media for the propagation of electromagnetic fields. As such, a shortcoming associated with using silicon-on-insulator substrates with oxide-filled trenches, is that virtually no RF isolation is provided.
It can therefore be seen that a method and apparatus for providing RF and DC shielding is desirable. More particularly, what is needed is a method and structure which isolates various analog and digital circuits which combine to form an integrated circuit, from each other.